This application is based on and hereby claims priority to German Application No. 102 01 322.5 filed on Jan. 15, 2002, the contents of which are hereby incorporated by reference.
The invention relates to a switching device for superconducting technology having a switching path which has a superconducting material, a mechanism for cooling the switching path to a predetermined operating temperature and a mechanism for exceeding at least one of the critical values of the superconducting material in a controlled manner to change the switching path from the superconducting state to the normally conductive state. A corresponding switching device is disclosed in DE 35 25 938 A1.
Superconducting switching devices have been developed for various superconducting technology apparatuses with conductors composed of traditional (metallic) superconductor material. Thus, for example, a switching device is known for a flow pump for excitation of a superconducting magnet (see, for example xe2x80x9cIEEE Transactions on Magneticsxe2x80x9d, Vol. 32, No. 4, July 1996, pages 2699 to 2702). Switching devices such as these are also provided for short-circuiting of magnetics such as those for MRI apparatuses or for energy storage devices. Corresponding switching devices generally have a switching path which is formed by a superconductor and is cooled to a predetermined temperature. This switching path can be changed from the superconducting state to the normally conductive state by deliberately exceeding at least one critical value of the superconductor material that is used, in particular the critical temperature or the critical magnetic field, or both. The switching device which is disclosed in the abovementioned DE-A document is of this type.
With the development of the new high TC superconductor materials (HTS materials) which are based on metal oxides, conductors for superconducting apparatuses are now also available which allow operation at considerably higher temperatures than is possible for the traditional superconductor materials. Superconducting switching devices have also been designed for apparatuses such as these. For example, U.S. Pat. No. 5,350,739 A discloses a microwave switch with a switching path composed of HTS material, which is located between two conductor parts composed of the appropriate HTS material. The switching path has associated heating device, to produce a transition from the superconducting state to the normally conductive state. A switching device which is disclosed in JP 11-340533 A is also of the corresponding type. In the switching device which is disclosed in U.S. Pat. No. 5,805,036 A and which uses HTS material, the switching process is produced by a magnetic field.
Some apparatuses with HTS conductors are intended to be operated below the temperature of about 77 K at which nitrogen becomes liquid. Switching devices with superconductive switching paths which can be changed from the superconducting to the normally conductive state at short notice may also be required for apparatuses such as these. Until now, the same HTS material as that used in the superconducting apparatus which is connected to it has generally been provided as the superconducting material for corresponding switching paths. The HTS material is in this case normally applied in the form of a layer to a mount which is suitable for this purpose.
However, it has been found that a number of problems can occur in corresponding embodiments of HTS switching paths, in particular such as
thermal inertia resulting from the relatively large mass of the mount or substrate that is used for the HTS material and owing to the fact that there may be a relatively large difference between a chosen operating temperature of, for example, below 40 K and a critical temperature of the HTS material of about 90 K or more,
difficulty in making contact owing to the sensitivity of the layer composed of the HTS material that is used and of a thin covering layer which can be applied to this layer and is composed of a contact-making material such as gold,
the risk of burning through excessively thin layers,
relatively complex material production, and
difficulty in coupling the switching device, such as a thermal heater to the HTS layer which is to be switched.
An aspect of the present invention is therefore to refine the switching device having the features mentioned initially such that the abovementioned difficulties are at least partially reduced.
According to the invention, this aspect may be achieved by providing magnesium diboride (MgB2) as the superconducting material for the switching path.
The superconducting compound (MgB2) has a critical temperature TC of approximately 39 K (see xe2x80x9cNaturexe2x80x9d, Vol. 410, Mar. 1, 2001, pages 63 and 54). This material has been identified as being particularly highly suitable for a switching device for superconducting technology with an operating temperature of below 40 K, in particular when operation is envisaged with cooling without the use of any coolant. Wires or thin layers can be produced from the cited material without any problems, in order to be used as the switching path. It is also advantageous that the known superconducting material or a sheath material surrounding it can be made contact with without any major difficulties.
For example, a wire with a round or rectangular cross section can be provided, in particular, for the superconducting switching path, which has a superconducting core composed of magnesium diboride, and has a non-superconducting metallic sheath. In this case, the sheath may advantageously be composed of iron or steel.
It is particularly advantageous for the mechanism for exceeding at least one of the critical values of the superconducting material in a controlled manner to be a control winding which surrounds the switching path. A control winding such as this allows the superconducting material to be subjected to a sufficient temperature increase and/or to a magnetic field in order to change, in a simple manner, from the superconducting state to the normally conductive state.
If the cooling system is designed without the use of coolant, a cooling machine can advantageously be thermally coupled to those parts of the switching device which need to be cooled.
Furthermore, to bound the switching path, it is advantageous for this switching path to be arranged between the heat sinks. This prevents undesirable broadening of the switching area of the superconducting material. Solid blocks or other solid bodies composed of a highly thermally conductive material are suitable for use as the heat sinks, which advantageously make a highly thermally conductive contact with the superconducting material, for example surrounding it.